Effect of the flow structure of pulsating turbulent flow in a duct with a double 90° bend on convective heat transfer at the wall

We evaluated the heat transfer characteristics of pulsating flow in an S-shaped double-bend pipe, which have been insufficiently evaluated experimentally and numerically. Conditions were set at Rem = 60,000, Dn = 38,000, and Wo = 33-80, corresponding to those of an engine exhaust pipe. The Nusselt number was evaluated from measurements of the wall temperature and internal fluid. Heat transfer characteristics were evaluated by performing a validated conjugate heat transfer simulation, and a large eddy simulation was conducted for the pulsating flow. The Nusselt number was assessed as higher than that of the steady flow under most pulsating conditions, but was lower than that of the steady flow at Wo = 42 and 46, confirming that pulsation suppressed heat transfer. This is the opposite of the heat transfer enhancement due to residual turbulence during deceleration observed in previous studies on straight pulsating flow at Wo = 42 and 46. The investigation of the mechanism of heat transfer suppression at Wo = 42 and 46 using numerical simulation revealed that the effect of heat transfer promotion by bending is lost as a result of the decrease in circumferential velocity of the Dean vortex during deceleration.